Telephone signals are provided to subscribers (customers) through the public switched telephone network. The subscriber portion of the network has two wires known as tip and ring, which carry the ring signal as well as the information being transferred. The bandwidth of the network is approximately 300 Hz to 3.3 kHz. Necessarily, any terminal equipment connected to this network must meet certain specifications in order to function properly.
To connect subscriber equipment such as data modems, facsimile machines, (non-cellular) portable telephones, speaker telephones, and message answering machines to the analog public switched telephone network, one must provide an interface or data access arrangement to bridge any incompatibilities between the network and the subscriber equipment. In addition to complying with network protocols, since the subscriber equipment are four-wire devices with separate transmit and receive pairs, the interface must separate the analog signals on the network into discrete transmit and receive signals (and vice versa). Finally, the interface must electrically isolate the telephone network from the subscriber equipment.
The interface must be transparent, so that from the vantage point of the central office, the line appears to be terminated in a conventional analog telephone set. Thus, the interface must simulate the D.C. continuity indicating the "off hook" condition when a call is received or placed. Typically, the separation function is performed by a circuit contained within the interface, called a 2-to-4 wire transhybrid.
For applications such as small portable computers and data entry devices, an interface having minimal volume and weight is ideal. However, the size cannot be arbitrarily reduced without impacting the performance. For example, some interface methods use isolation transformers to provide the required D.C. separation between the network and the subscriber, which offer wide bandwidth and transparency. However, transformers suited for this application suffer from being physically large, heavy, and costly, and therefore represent a poor choice for portable devices. If the transformer is eliminated from the interface, a substitute providing the requisite performance is needed.
An ideal interface or data access arrangement should offer a flat frequency response, constant group delay, extremely minimal amplitude and frequency distortion, and reflect the proper line impedance. It would be desirable to provide an interface not requiring the traditional line transformer but yet offers the same or a superior level of performance.